International patent applications PCT/GB94/01193, PCT/GB94/01194, PCT/GB94/01195, published as WO 94/28306, WO 94/28307 and WO 94/28308 respectively, and deriving from one of the inventors of the present application, describe a compressor which provides a number of improvements in the art. Reference should be made to these applications, hereinafter called "the earlier applications", for background discussion of such compressors and for details of the compressor described in those applications.
The present invention seeks to provide some modification and improvement of the compressor of the earlier applications. Particularly the present invention is concerned with the problem of reducing wear, particularly uneven wear, of the sliding surfaces of the reciprocating piston. In addition the invention is concerned with improving the ease of manufacture and assembly of the compressor.
It is well known in electromagnetic reciprocating compressors to provide a helical compression spring which causes the return stroke of the piston. In the earlier applications, particularly WO 94/28306, it is described how inherent defects in the compression spring or misalignment in its mounting can cause the spring to apply unsymmetric force to the piston, resulting in uneven wear.
It is known from GB-A-2241287 in a double-acting electromagnetic compressor, in which the piston alternately compresses gas in two opposed working chambers, to mount two compression springs which restore the piston to a neutral position inside the respective working chambers.
It is known from EP-A-509660 to locate a return spring around the piston and partly in the cylinder of the compressor outside the working chamber of the cylinder.